From the perspective of the daylily plant (Hemerocallis spp.), the emergence of Daylily Rust, caused by the fungus Puccinia hemerocallidis, represents a significant physiological stressor. This pathogen is an obligate parasite, meaning it can only survive and reproduce on living daylily tissue. The introduction of this rust to the United States in the early 2000s created a new challenge for these popular perennial plants, disrupting their natural growth cycles and compromising their aesthetic and physiological vigor. The interaction is a classic host-pathogen relationship where the fungus seeks to extract nutrients at the expense of the plant's health.
The first visible signs of infection are critical indicators of the plant's distress. Initially, the upper surface of the leaves develops small, yellow flecks or streaks. These are chlorotic areas where the fungus is disrupting chlorophyll production. The most definitive symptom, however, appears on the underside of the leaves and on the scapes (flower stalks). Here, the plant's tissue reacts to the fungus by forming raised, pustules that rupture to release masses of spores. These pustules are typically a rusty orange or cinnamon-brown color, giving the disease its common name. From the plant's viewpoint, this spore production is not a voluntary act but a consequence of the fungus colonizing the leaf's interior and manipulating its resources for reproduction. Severe infections cause leaves to yellow, wither, and die prematurely, drastically reducing the plant's ability to photosynthesize and store energy for the next season.
The life cycle of the rust fungus is intimately tied to the daylily's own growth. The fungus overwinters as dormant mycelium or teliospores in infected plant debris or on evergreen daylily leaves in warmer climates. In spring, as the daylily begins its active growth, the fungus becomes active. It penetrates the leaf tissue directly through the epidermis or through natural openings like stomata. Once inside, it forms haustoria, specialized structures that invade the plant's cells to siphon water and nutrients. This direct theft of resources weakens the plant, leading to reduced vigor, smaller scapes, and diminished blooms. The rust thrives in the same conditions that daylilies love for growth—warm temperatures (70-80°F) and high humidity or leaf wetness—making the plant's ideal environment also ideal for its attacker.
Prevention focuses on creating an environment and cultivation practice that supports the daylily's natural defenses while minimizing opportunities for the pathogen. Key strategies include:
4.1. Cultivar Selection: The plant's genetic makeup is its first line of defense. While no daylily is completely immune, many tetraploid cultivars and certain diploid varieties exhibit strong resistance. Choosing these resistant plants is the most effective long-term strategy.
4.2. Cultural Practices for Plant Health: Maintaining plant vigor is paramount. This involves proper spacing to ensure good air circulation, which helps leaves dry quickly and creates a less favorable microclimate for spore germination. Watering at the base of the plant, rather than overhead, keeps foliage dry. Providing adequate nutrition without over-fertilizing with nitrogen, which can produce soft, succulent growth more susceptible to infection, is also crucial.
4.3. Sanitation: From the plant's perspective, removing infected leaves and cleaning up all plant debris in the fall is a form of parasite removal. This eliminates the primary source of overwintering fungus, reducing the inoculum that will threaten new growth in the spring.
When prevention fails, control measures aim to alleviate the stress on the plant. Upon first sight of symptoms, the immediate removal and destruction of infected leaves can slow the disease's spread within the plant and to neighboring plants. For recurring or severe infections, the application of fungicides may be necessary. To the plant, these fungicides are a protective shield; contact fungicides act as a barrier on the leaf surface to prevent spore germination, while systemic fungicides are absorbed by the plant and work from within to stop the fungal growth. It is essential to rotate fungicide classes to prevent the rust fungus from developing resistance, ensuring these tools remain effective for the plant's protection.
